pack-objects: flip the use of GIT_TEST_PACK_SPARSE

pack-objects: flip the use of GIT_TEST_PACK_SPARSE

The environment variable GIT_TEST_PACK_SPARSE was previously used
to allow testing the --sparse option for "git pack-objects" in
the test suite. This allowed interesting cases of "git push" to
also test this algorithm.

Since pack.useSparse is now true by default, we do not need this
variable to _enable_ the --sparse option, but instead to _disable_
it. This flips how we work with the variable a bit.

When checking for the variable, default to a value of -1 for
"unset". If unset, then take the default from the repo settings,
which is currently 1. Then, the --[no-]sparse command-line option
will override either of these settings.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>